1. Field of the Invention
The present invention relates to the field of measurement instrumentation, and more particularly to automated particle quantifying apparatus and method that can provide estimation of the quantity of particles present in a volume or on a surface, such as particles collected from the surface of a moving web by a suction extractor.
2. Description of the Related Art
In the past years, new measurement instrumentation aimed at detecting and quantifying particle found within or onto a variety of media has been continuously developed either to better comply with the requirements of specific applications or to increase processing speed as often required by real-time industrial applications. Among those industrial fields that have experienced significant instrumentation development is the paper manufacturing industry, for the purpose of detecting and quantifying the small particles that form onto both surfaces of a paper web during manufacturing and due to partial or total fiber separation from the paper web material. This phenomena, commonly known as “linting”, involves particles that are generally known to consist of loose and weakly bound fibers originating from ray cells, shives and other fine matter present on the surface of the paper, characterized by a relatively low specific area which does not provide sufficient bounding to the well-adhered fibers constituting the paper web. Although linting generally does not imply significant modification of physical properties of the paper material, it is known to be at the origin of important problems at the subsequent printing process due to lint fiber separation that occurs when the paper is passing through printing equipment. More specifically, during the printing process, the particle or lint material adheres to the printing blanket and interferes with ink transfer thereto, thereby causing gradual dogging of the printing machines while reducing printing quality. Printing interruption that is rendered necessary to perform cleaning or printing machinery contributes to significantly reduce the efficiency of the printing process while increasing printing cost. In an attempt to comply with paper quality requirements from the printing industry which imposes limited levels of linting, the paper manufacturing industry has proposed some techniques to detect and quantify linting on the production site so as to adjust the parameters of pulp and paper production processes accordingly, which parameters relate to raw wood chips blend from resinous and hardwood species, pulp characteristics and processing steps such as sieving and purification steps as well as paper manufacturing machine types.
A known technique for linting measurement consists of using a small commercial printing press installed at the paper-manufacturing location and operated under controlled conditions representative of normal production printing conditions. Following a predetermined operation period, lint particles are collected from the ink train and blanket of the printing press and are then weighed after drying to provide an estimation of the paper's linting propensity. Such off-line technique being expensive and time-consuming, it has been held impracticable to be used as a routine quality control operation.
A known online paper web surface cleaner-tester is disclosed in U.S. Pat. No. 4,905,500 issued to Mason, by which lint material is freed from the surface of a paper web traveling pass a material pickup head having an air bearing surface through and along which an air stream is passed creating a negative pressure causing the air bearing surface to be held in close proximity to the web while causing lint material to be freed from the web surface and carried in the air stream. The material pick-up head uses a vacuum at its inlet opening located relative to the air-bearing surface such that the freed material is sucked into the inlet and transported toward a particle-collecting filter provided with a device for measuring the pressure drop across the filter to determine a preset amount of material captured. The time required to capture the preset amount of material is measured to provide an indication of the paper's linting propensity. Alternatively, the pressure drop after a preset time maybe measured to provide an equivalent estimation. Although representing an improvement over the known off-line technique using a small printing press, the on-line paper web surface cleaner-tester as taught by Mason cannot allow linting measurement on a continuous basis since the filter must be manually changed after being loaded with particles following a single test. Although the predetermined pressure drop value can be set so as to limit the entire particle capturing time to less than one minute, the time required to manually substitute a loaded filter with a new, particle-free filter for performing a following test could hardly be restrained within a reasonable limit so as to provide reliable and useful continuous linting estimation.
In an attempt to improve the measurement rate of paper's linting propensity, an automated dust measuring apparatus has been proposed by Allen et al in U.S. Pat. No. 4,898,037 which uses a cylindrical roll provided with an elastomeric covering adjacent a metal feed cylinder to form a nip through paper sheets are conveyed, whereby dust released from the surface of the paper sheets in contact with the elastomeric covering surface is transferred thereto. The apparatus comprises an electrical drive motor for rotating the cylindrical roll at a predetermined speed and a mechanism to bring the roll at a working position wherein a predetermined pressure is applied to the paper sheets between the roll and the feed cylinder. When one revolution of the roll has been completed, the mechanism is operated to disengage the roll for opening the nip. The apparatus further includes an optical sensing device having a light source and a detector with its associated electronics for measuring the change in reflectance of the elastomeric covering surface of the roll. The elastomeric covering surface is conveniently colored black, for changing to a greyish tone with a buildup of dust. A computer is provided to control the apparatus and to compare the reflectance measurement signal with a previously obtained value for the reflectivity of the clean roll surface considered as a reference. The roll positioning mechanism is preferably a rotary mechanism capable of moving the roll between a dust transferring position wherein it forms with the adjacent feed cylinder a nip contacting the paper sheet passing there between, to a roll cleaning position wherein dust adhered to the surface elastromeric covering is being dislodged through the action of water or cleaning fluid that is circulated into an out of a bath provided at the cleaning position. Disposed at an intermediate position is a hot air blower arranged to blow hot air onto the roll to remove cleaning fluid remaining thereon. Apart from the risk of paper damage due to contact thereof with the roll and feed cylinder at the transfer nip, such automated apparatus cannot allow dust measurement while the capturing roll is brought to the cleaning position and then after to the drying position, thereby significantly limiting dust quantifying rate.
It is therefor still desirable to provide automated apparatus and method for efficiently estimating the quantity of particles collected by a suction extractor and more particularly for estimating the quantity of particles on the surface of a web such as paper web traveling through a collecting area, which advantageously obviate the above-mentioned limitations suffered by known prior art techniques.